The present invention relates to improved methods for the processing of carbon foams, carbon fibers, carbon ceramic composites and C/C composites and more particularly to methods for achieving oxidation/stabilization of such materials in significantly shorter periods of time than has been possible with prior art techniques and to carbon based products produced thereby.
Current carbon materials are derived mainly from either petroleum or coal. Petroleum-derived pitch and graphitic carbon have demonstrated superior mechanical, electrical and thermal properties. Coal-derived carbon is more difficult to process and generally has less material uniformity and lower performance, i.e properties than those derived from petroleum. Most of the petroleum companies produce pitch, a carbon precursor, as a by-product of the refining process. Mitsubishi Chemical Co. is one of the major producers of high performance mesophase pitch. Commercial sources of coal-derived pitch include Athabasca (or Alberta) pitch, Pittsburgh pitch, etc. The chemistry and processing of carbon is well known and has been discussed in numerous textbooks and publications.
High performance carbon fibers are conventionally produced from polyacrylonitrile (PAN). After spinning, the post-processing steps convert the resin fibers into carbon. Rayon is another carbon precursor but the properties of carbon materials produced therefrom are generally considered inferior to those obtained from pitch, petroleum, or PAN.
Crystalline carbon has excellent structural and propulsion properties including mechanical and thermal properties, retention of excellent mechanical properties at elevated temperatures, light-weight, excellent dimensional stability over broad ranges of temperature, and zero-outgassing. Because of their exceptional performance to weight ratio, carbon and carbon-carbon composites can play a unique role in structural components that fly or move including aircraft, space vehicles, and surface vehicles. A major obstacle for widespread use of carbon and carbon-based composites is the cost. High cost is due largely to the long post processing steps required in the fabrication of these materials. Carbon-based materials require several processing and post-processing steps at very high temperatures and for long periods of time, generally expressed in terms of tens of hours to obtain the highly desirable properties referred to hereinabove.
It is the costs associated with these long processing times that contributes significantly to their very high cost. The post-processing steps are, however, the key factors that determine the desirable mechanical, thermal and electrical properties of the carbon materials.
Another major challenge is the post-processing of thick section carbon structures where oxidation/stabilization is dependent upon diffusion of oxygen to the interior portion of the structure. In such structures, it is very difficult to get enough oxygen to the interior of the structure during oxidation/stabilization to provide a homogeneous structure. This can result in unacceptable non-uniform material properties and defects in the final structure. It is, therefore, extremely important to develop rapid and effective post-processing techniques for carbon-based materials from both an economic and a performance point of view. It is also very important that the technique can be applied to both thin and thick section structures.
High performance carbon-based materials are available in either dense or low-density form. The process for making dense or porous carbon from pitch typically involves preparation of a mesophase (liquid crystalline) pitch from residues and converting it into fibers or foams. As the precursors are spun into fibers, the large molecules and crystallites become aligned with the fiber axis. The subsequent oxygen stabilization step forms cross-links. Carbonization is conducted at temperatures around 1000xc2x0 C., and graphitization at between 2200 and 3000xc2x0 C. Among these steps, the stabilization step is generally the rate-limiting step in the process as it can typically take 24 to over 48 hours. While many options exist for stabilization (e.g., reaction with HNO3), the method practiced most broadly is thermal oxidation in circulating air or a mixture of air and nitrogen. The oxidative stabilization reaction is believed to be oxygen diffusion limited. Two stabilization mechanisms are posited as possible: adding oxygen to an organic molecules raise the boiling or melting point (e.g., phenol boils at 182xc2x0 C. vs. 80xc2x0 C. for benzene) or, alternatively, oxygen may promote cross-linking between molecules. PAN and rayon are processed in ways similar to pitch.
Examination of the fundamental steps involved in stabilization has revealed that the time consuming stabilization is apparently due to the slow delivery of oxygen from the bulk to the internal region of the pitch or carbon structure. To homogeneously stabilize foam, oxygen needs to diffuse into the whole carbon mass. For very short oxidation times or for large pieces of carbon precursor, oxygen does not reach the center of the material, and such stabilization tends to introduce a non-uniform microstructure during carbonization. Oxygen permeation through the skin (outer layer) thickness of such compositions is believed to vary as the square root of time, indicating a diffusion controlled process of stabilization. The outer part (nearest the outer surface of the carbon material) often appears to be over oxidized, while the core (at the center of the carbon material) is insufficiently stabilized/oxidized and allows porosity formation and higher mobility of the molecular structure during pyrolysis (carbonization/graphitization). A carbon material with such a profile of stabilization does not have the superior mechanical properties preferably sought in such materials.
The present invention provides an enhanced method for the post processing, i.e. oxidation or stabilization, of carbon materials including, but not limited to, carbon foams, carbon fibers, dense carbon-carbon composites, carbon/ceramic and carbon/metal composites, which method requires relatively very short and more effective such processing steps. The introduction of an xe2x80x9coxygen spill over catalystxe2x80x9d into the carbon precursor supplies required oxygen at the atomic level and permits oxidation/stabilization of carbon materials in a fraction of the time and with a fraction of the energy normally required to accomplish such carbon processing steps. The products of these improved processes are also described and characterized.